During the production of olefin polymers in a commercial reactor it is often necessary to transition from one type of catalyst system producing polymers having certain properties and characteristics to another catalyst system capable of producing polymers of different chemical and/or physical attributes. Transitioning between similar Ziegler-Natta catalyst systems, or compatible catalyst systems, for instance, generally takes place easily. However, where the catalyst systems are incompatible or of different types the process is typically complicated. For example, when transitioning between two incompatible catalyst systems such as a Ziegler-Natta catalyst system and a metallocene catalyst system, it has been found that some of the components of the Ziegler-Natta catalyst system act as poisons to the metallocene catalyst system. Consequently, the components of the Ziegler-Natta catalyst system prevent the metallocene catalyst system from promoting polymerization.
In the past, to accomplish an effective transition between incompatible catalysts, the first catalyzed olefin polymerization process was stopped by various techniques known in the art. The reactor was then emptied, recharged and a second catalyst system was introduced into the reactor. Such catalyst conversions are time consuming and costly because of the need for a reactor shut-down for an extended period of time during transition.
It would be highly advantageous to have a process for transitioning between incompatible catalysts, without the need for halting the polymerization reaction, emptying the reactor to rid it of the original catalyst system and restarting the polymerization reaction with another catalyst system. In addition, it would be advantageous if the process for transitioning could reduce the amount of off-grade material produced during the transition process, reduce the transition time, increase the robustness and stability of the transition process and avoid the need to open the reactor to charge the seed bed.